A conventional traffic signal employs a power supply and control electronic module located inside the traffic signal head. This configuration has the following limitations:
The conventional power supply and control module are located in an environmentally unfriendly location. The signal head is exposed to direct sunlight without proper ventilation, meaning it is exposed to extremes in temperature. Worse, if the power supply and control module fails, traffic lanes must be closed and the repair made using a “bucket truck” to reach the signal head.
Since the conventional control module is located in the signal head, information must be communicated from the control module to the traffic signal controller mounted in an electrical cabinet beside the roadway. To accomplish this, a separate communications line must be installed, or the information must be superimposed on the existing traffic signal electrical wires, or the information must be transmitted via a wireless method.
Since a high-frequency switching regulator is enclosed in a metal electrical cabinet at the street corner, the radiated electrical noise created by the switching circuitry must be shielded from the radios of passing motorists by the metal electrical cabinet, and is not placed overhead with high-frequency radio emissions.
Since conventional traffic signal control is configured to detect malfunctioning incandescent bulbs by measuring signal head voltage, measuring the signal head voltage of an LED signal does always detect a malfunction, as the LED gradually loses light output, even with proper voltage levels applied.
Since the conventional control module is located in the signal head, and communications from the signal head to the traffic signal controller is generally not available, or not affordable, the conventional signal head responds to a calculated end-of-life by breaking a fuse to emulate a “burned-out” incandescent bulb. This method has two disadvantages:                1. Abrupt loss of traffic signal causes an unsafe condition for drivers        2. Historically, the method to emulate a “burned-out” bulb frequently malfunctions and causes the signal to prematurely fail.        
Traditionally, the old-style traffic signal bulb filaments would simply burn out at the end of the bulb life. Special monitoring circuitry connected to the wire feeding power from the traffic signal controller to the signal head senses the voltage across the bulb. If the bulb filament is intact, the voltage measured across the bulb is essentially zero. If the filament is burned-out, the lamp switch leakage is no longer connected through the filament, and the voltage across the bulb is large, indicating the dangerous condition to the Traffic Control Center. This sensor might also place the intersection into FLASH RED in the opposing direction, to insure motorist safety. The Traffic Control Center would then schedule a service call to replace the bulb.
Currently, the incandescent bulbs of traffic signals are being replaced by LED light sources, with the advantage of much lower power and longer life. Because incandescent bulbs emit tungsten light, consisting of a broad color spectrum, only a small portion of the light is passed through a color filter to the driver. LEDs emit monochrome light. For example, a RED LED emits RED light, meaning that the power to produce only light of the desired color is much less. Because LEDs do not operate on the normal power line voltage (120 VAC, 60 Hz in the US, for example), a power supply is embedded in each signal head to convert the power line voltage to the lower voltage and current required by the LED light source. However, because LED light sources do not “burn out” as do light bulbs, another problem is created. As the LED light source ages, its light output gradually decreases, to the point of creating a dangerous condition. Worst, after the LED light output has reached a dangerously low level, no corresponding loss of signal voltage or current alerts the traffic signal controller to the danger. To counteract this problem, a control module is installed in each signal head. Different methods are used by the control module to sense the end-of-life for the LED light source. In one method, the LED light source brightness is measured by the control module using a photo sensor, such as a photo diode, photo transistor, or cadmium sulfide cell. As the light output falls with age or temperature, the control module increases power to the LED light source to compensate.
Once the control module determines that the LED light source has reached the end of its life, different methods are used to inform the Traffic Control Center, among them:                1. A fuse is installed in the signal head, in series with the LED light source. Once the control module determines that the LED light source has reached the end of its life, the control module will “blow” the fuse, simulating a bulb burning out. The traffic signal controller senses the loss of signal head power and indicates the event to the Traffic Control Center.        2. A communications link is added that connects the control module of each signal head to the traffic signal controller. Once the control module determines that the LED light source has reached the end of its life, the control module will communicate this information to the traffic signal controller and the Traffic Control Center via the communications link. This communications link might take the form of a separate set of wires, a signal superimposed on the power line to the signal head, or wireless, such as radio or infrared.        
Thus, the conventional traffic signal has disadvantages, with some of the disadvantages listed below:
Each signal head includes a power supply, which adds expense, is prone to failure and is located overhead, where servicing and replacement are inconvenient at best and dangerous to the motorist at worst.
To maintain LED signal efficiency, the power supply installed in each signal head employs a switching regulator. This type of regulator increases or decreases the LED light output by switching the LED light source ON and OFF at a rapid rate (usually about 20,000 times per second). The light output is controlled by varying the amount of ON time relative to OFF time (duty-cycle). While very efficient, this method naturally transmits this switching frequency into the air, causing potential interference with radios and emergency communications. To counteract this problem, various noise-suppression and shielding techniques are required.
The end-of-life indication method of “blowing” a fuse provides no prior warning, meaning that the fuse may blow in the middle of rush hour, disabling a vital traffic signal. This method could endanger the public until the signal is replaced.
The end-of-life indication method of “blowing” a fuse frequently malfunctions and “blows” prematurely, especially during conditions of lightning surges.
The end-of-life indication method employing communications adds cost and complexity, including the possible installation of additional wires for communications lines.
Thus, there is a need to eliminate the power supply and control module in the signal head of a traffic signal.